1. Field of the Invention
The present invention relates to a liquid crystal display device and, more specifically, to a liquid crystal display device which improves the orientation of liquid crystal molecules on a substrate, thus ensuring a high display quality, and to a method of producing the same. The present invention also relates to a liquid crystal display device capable of being operated at high speed such as for field-sequential driving directed to a moving picture display, and to a method of producing the same.
2. Description of the Related Art
Liquid crystal display devices are widely used in personal computers, liquid crystal TVs, car navigation systems, digital cameras, video cameras and cellular phones. In notebook PCs and liquid crystal TVs, further, it has been strongly desired to widen the visual angle, i.e., to improve visual angle characteristics and to improve the image fineness to meet an increase in the screen sizes. Liquid crystal display devices, in general, have a structure in which liquid crystals are sealed between a pair of insulating substrates, such as glass substrates, in such a manner that the liquid crystal molecules thereof are oriented in a predetermined direction, and an orientation film is formed on the respective substrates on the side of the liquid crystals. As a material of an orientation film, there is usually used a polyimide or a polyamic acid (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 64-4720). The liquid crystal molecules are highly oriented by subjecting the orientation film formed on the insulating substrate to an orientation processing for setting a pretilt angle which is a contact angle between the liquid crystal molecules and the insulating substrate. Further, there have been suggested many liquid crystal devices to improve the viewing angle characteristics and screen fineness. For example, there has been proposed a split orientation panel structure in which a plurality of liquid crystal molecules having different orientation directions are included.
Conventional liquid crystal display devices are usually produced through a step of forming an oriented film, a step of orientation processing, a step of assembling substrates and a step of sealing liquid crystals as schematically illustrated in a flowchart of FIG. 1. These production steps will be further described. First, a material for an orientation film is applied onto the insulating element or CF substrate that has been washed and is baked to form an orientation film. Then, rubbing is conducted for setting a pretilt angle by rubbing the surface of the orientation film with a buffing cloth. Dust and dirt as well as static electricity formed by rubbing are removed by washing and drying to thereby effect the orientation processing. After the orientation processing has been completed, a sealing material (usually an adhesive) is printed onto the element substrate and, then, a spacer material is applied or distributed (hereinafter, also referred to as “sprinkled”) to obtain a gap relative to the filter substrate. Next, the element substrate and the filter substrate are stuck together to assemble a liquid crystal substrate. Then, a liquid crystal is injected through an injection port of the insulating substrate obtained as described above, and the injection port is sealed. One or two polarizing plates are mounted on the filter substrate to complete a liquid crystal panel having a liquid crystal sealed between a pair of substrates having the orientation films. After a series of these steps are completed, a predetermined circuit is assembled around a liquid crystal panel to thereby complete a product such as a notebook personal computer.
Among the above series of production steps, a rubbing method of rubbing the surface of the orientation film with a buffing cloth is employed in the step of orientation processing for setting a pretilt angle which is a contact angle between the liquid crystal molecules and the insulating substrate. Therefore, the physical contact and friction generate dirt and dust, cause damage to the orientation films, contaminate the surface of the liquid crystal layer due to static electricity, cause thin-film transistors (TFTs) formed on the insulating substrate to be electrostatically broken down and cause a drop in the production yield. In order to solve these problems, however, after-treatments such as washing and drying must be conducted inevitably resulting in an increase in the number of the processing steps. Further, since only one or two liquid crystal panels are formed on a piece of insulating substrate accompanying an increase in the screen size, the production yield assumes only two values of 0% and 100% or assumes only three values of 0%, 50% and 100% depending upon the presence of defect. As a high degree of image fineness is demanded, further, the number of display elements formed on one liquid crystal panel becomes, for example, 640×480 dots. In the case of a three-color display, the dots could amount to 920,000 elements and a drop in the yield due to the occurrence of defects could become a serious problem.
In order to solve this problem, there has been proposed an optical orientation method of setting a pretilt angle in the orientation film by utilizing light without relying upon the rubbing method which sets the pretilt angle by physical contact with the surface of the orientation film (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-5747). As described above, the pretilt angle represents the contact angle between the insulating substrate and the liquid crystal molecules. Accordingly, the direction in which the pretilt angle is set is usually called a tilting direction.
An example of the optical orientation method will be described with reference to FIGS. 2A to 2C.
Referring to FIG. 2A, a material of the orientation film such as polyimide or polyamic acid is applied onto an insulating substrate 51 such as a glass substrate and is baked to form an orientation film 52. Then, the insulating substrate 51 is vertically irradiated with ultraviolet rays 54a polarized in a vertical direction to effect pre-exposure which is for adjusting the physical properties of the orientation film material. Next, as shown in FIG. 2B, the insulating substrate 51 is irradiated, at a predetermined angle α°, with ultraviolet rays 54b of which the direction of polarization is changed by 90 degrees, whereby a pretilt angle a inclined in the direction of irradiation of ultraviolet rays is set in the orientation film 52, and liquid crystal molecules 56 are oriented in the same direction and at the same angle as the pretilt angle as shown in FIG. 2C. According to the above orientation processing based on the optical orientation method, no step is required for effecting the rubbing, washing and drying, solving the above-mentioned problem.
This optical orientation method, however, still involves a problem. For example, if it is attempted to realize a split orientation panel structure which may make it possible to improve the visual angle characteristics of the liquid crystal pane and the image fineness of the screen by applying the above optical orientation method, a plurality of pretilt angles must be set in the regions of the orientation film depending upon the orientation directions of the liquid crystal molecules by the irradiation with ultraviolet rays polarized in different directions of irradiation. In order to set a plurality of pretilt angles on the same orientation film, it becomes necessary to prepare a plural kinds of optical masks for transmitting and shutting off the polarized ultraviolet rays and to irradiate polarized ultraviolet rays having different angles of irradiation a plural number of times by using the optical masks, requiring a very long time for the orientation processing.
When the orientation film is formed on the substrate, further, not only must the number of production steps be increased but also large dedicated production equipment must be installed such as a printer for printing the orientation film material and a baking apparatus for the orientation film, inevitably resulting in an expansion of the production side and a great increase in the cost of production.
Further, in order to decrease the number of production steps and to increase the yield, there has also been proposed, in Japanese Unexamined Patent Publication (Kokai) No. 11-95221, a method of producing liquid crystal display elements by sealing a liquid crystal composition containing an orientation assistant of a photo curable high-molecular resin in a liquid crystal cell between the glass substrates, leaving the liquid crystal composition to stand, and causing the orientation assistant to be adsorbed by the surface of the substrate by utilizing the surface energy of the glass substrate thereby to form an orientation film.
Turning again to the structure of the conventional liquid crystal display devices, there has been suggested a liquid crystal display (LCD) using an active matrix, and as such a LCD, there has been widely used a liquid crystal display device of a TN mode in which a liquid crystal material having a positive dielectric anisotropy is horizontally oriented to the surface of the substrate in a dark state and is twisted by 90 degrees relative to the opposing substrate.
The liquid crystal display device of the TN mode has a problem of poor visual angle characteristics, and study has been extensively conducted in an attempt to improve the visual angle characteristics. As an alternative system, there has been developed an MVA (multi-domain vertical alignment) system according to which a liquid crystal material having a negative dielectric anisotropy is vertically oriented, and the direction of tilting the liquid crystal molecules is controlled in a plurality of directions when a voltage is applied by utilizing protuberances and slits formed on the substrate surfaces without rubbing the orientation film, succeeding in greatly improving the visual angle characteristics.
However, in order to vertically orient (horizontally orient) the liquid crystal material, it is essential to form an orientation film by using a polyimide or the like, not only in the liquid crystal display devices of the TN mode but also in the MVA system. Formation of the orientation film requires a printing step, a baking step, a washing step and the like, which are the major factors of preventing a reduction-in the production steps and a reduction in the cost.
In addition, as a liquid crystal display device using an active matrix, there has been widely used a liquid crystal display device employing a nematic liquid crystal. In recent years, it has been required to provide a liquid crystal display device such as for a field sequential drive type adapted to a moving picture display. This liquid crystal display device requires high-speed switching of liquid crystal molecules. Therefore, it has been urged to provide a liquid crystal display device having a higher response speed.
In the liquid crystal display device using a nematic liquid crystal, decreasing the cell thickness and increasing the pretilt angle are effective in increasing the response speed of the liquid crystal. Pretilting the liquid crystals is usually realized by rubbing an organic orientation film such as those of polyimide or polyamic acid with a cloth. However, it is difficult to obtain a uniform and large pretilt angle by rubbing. In general, a pretilt angle of about 6 to 7 degrees is an upper limit. Besides, the rubbing produces dust and dirt creating such a problem that elements are destroyed due to static electricity. It is therefore desired particularly for the active substrate that the orientation treatment is carried out by a method other than rubbing.
Further, a large pretilt angle can be effectively obtained by using an organic orientation film containing an alkyl group in large amounts and by increasing the density of the alkyl groups on the surface of the orientation film. When it is attempted to obtain a large pretilt angle by the above method, however, there arises a problem in that the shading becomes conspicuous due to dispersion in the pretilt angle on a plane, and stripes become conspicuous due to rubbing.
As an orientation method other than rubbing, there have been proposed a method of providing bank structures on the surface of the orientation film and an optical orientation method which has been described above with reference to FIGS. 2A to 2C. According to the method of providing bank structures on the surface of the orientation film, first striped banks are formed on the orientation film on one substrate, and second striped banks are formed on the orientation film on the other substrate in parallel with the first striped banks but being deviated from the first striped banks. The orientation of the whole liquid crystal molecules is controlled by utilizing the fact that the liquid crystal molecules between the first striped banks and the second striped banks have a property of being oriented vertically to these striped banks.
However, the method of providing the bank structures is applied to the liquid crystal display device of the vertical orientation type but is not applied to the liquid crystal display device of the TN type. Further, according to the optical orientation method, the orientation film of polyimide or polyamic acid is irradiated with ultraviolet rays to impart anisotropy to the surface of the orientation film thereby to orient the liquid crystal molecules. With the optical orientation method, however, the force for limiting the orientation is weak, and it is difficult to realize a large pretilt angle. With the optical orientation method, the pretilt angle is, for example, about 1 degree.
Moreover, as for the technology for controlling the orientation of the liquid crystal display device, Japanese Unexamined Patent Publication (Kokai) No. 5-173138 teaches dividing the orientation, and Japanese Unexamined Patent Publication (Kokai) No. 9-146096 discloses a liquid crystal display device in which the orientation film comprises striped vertical orientation regions and striped horizontal orientation regions that are alternately arranged, and is rubbed in the directions perpendicular to the vertical orientation regions and to the horizontal orientation regions.